Capacitors for use in electronic instruments such as portable telephone and personal computer are demanded to have a small size and a large capacitance. Among conventional capacitors, tantalum capacitors are preferred because of their large capacitance for the size and good performance. In these tantalum capacitors, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the weight of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The former method of increasing the weight of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the latter method of pulverizing the tantalum powder to increase the surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering and therefore, impregnation of the cathode agent in the later process becomes difficult. As one of means for solving these problems, a capacitor using a sintered body of powder of a material having a dielectric constant larger than that of tantalum is being studied. The materials having a larger dielectric constant include niobium.
Niobium is homologous to tantalum but is greatly different from tantalum in the characteristics as the material for capacitors. For example, if tantalum contains 10,000 ppm by mass of oxygen as impurities, the leakage current characteristics are greatly deteriorated, however, niobium has no such a problem and even if niobium has an oxygen content of tens of thousands of ppm by mass, the leakage current characteristics are scarcely deteriorated.
Capacitors manufactured using niobium as a raw material are, however, inferior in the voltage resistance characteristics to capacitors manufactured using tantalum as a raw material.
Known publications describing the relationship between the amount of impurity elements contained in the niobium powder and the capacitor performance include International Patent Publications WO00/49633 and WO00/56486. The former discloses that the capacitor performance such as specific leakage current of the capacitor can be improved by reducing the content of specific impurity elements such as iron, nickel and cobalt, to 100 ppm by mass or less, and the latter discloses that this effect can be attained by adjusting the carbon content to from 40 to 200 ppm by mass and the iron, nickel and chromium content to approximately from 5 to 200 ppm by mass. However, either publication does not disclose the relation between the chromium content and the voltage resistance characteristics of the capacitor.